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Thecodontia (meaning 'socket-teeth'), now considered an obsolete taxonomic grouping, was formerly used to describe a diverse "order" of early that first appeared in the latest Permian period and flourished until the end of the Triassic period. All of them were built somewhat like crocodiles but with shorter skulls, more erect pose and usually somewhat lighter. The group includes the ancestors of , , and , as well as a number of extinct forms that did not give rise to any descendants. The term thecodont is still used as an anatomical description of the tooth morphology seen in these species and others.
They constitute an evolutionary grade of animals, a "wastebasket taxon" for any archosaur other than a crocodilian, a pterosaur, or a dinosaur (any basal archosaur). Because the clade only recognises monophyletic taxa as natural groups, and because thecodonts are a paraphyletic group (they include among their descendants animals that are not thecodonts), the term is no longer used as a formal name by most , but it can still be found in older (and even fairly recent) books as a convenient shorthand for the basal archosaurs.
Robert Carroll, in his book Vertebrate Paleontology and Evolution (1988), replaces Pseudosuchia with Rauisuchia, Ornithosuchia (containing Ornithosuchidae and non-dinosaur, non-pterosaur Ornithodira), and the traditional category incertae sedis (of uncertain placement), while retaining the other three suborders. This is the last major textbook that still recognizes the taxon Thecodontia, as it uses a traditional Linnaean based taxonomy derived from that of Romer.
In his 1982 thesis on amniote phylogeny, Brian Gardiner attempted to define Thecodontia within a cladistic framework, containing crocodilians and haemothermata (mammals and birds), thus giving the old name to a new concept. All more recent cladistic studies (e.g., Jacques Gauthier 1986) have confirmed that Thecodontia as construed by Gardiner is a polyphyletic taxon, the members of which are not united by any synapomorphy. As the association of the name with the outdated concept proved to be very strong, it is now considered a historical term only, and its current usage has been abandoned.
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